But here’s the thing, conflicts among children sometimes escalate very quickly, and other times they accumulate over time. It might be useful to find out if that child has experienced similar issues with other friends, if there’s a pattern. I would suggest looking not only at your child’s feelings but also at this broader context.
Even the accuracy of what is said about the past can be questionable if first-hand information cannot be accessed. Rather than making assumptions about children’s behavior, have you tried looking a little closer at that friend? Children sometimes reflect, sometimes hide, the clue is there.
Questioning the accuracy of what is said about the past is important to a certain extent, but the real question is: Does your child feel comfortable and safe while spending time with that friend? The way to understand this is to observe a bit more carefully. It seems more sensible to me to watch what is happening in the environment without putting direct pressure on the kids.
If the child’s troubled past is not as you’ve heard, it would be more beneficial to learn about it from your own child’s perspective. Perhaps a different story is circulating among the children, and they may have derived a completely different meaning from it compared to the adults. Ask your child how they feel about that friend directly and request them to explain why they feel that way, let’s see what comes out.
Understanding a child’s own feelings is important, but sometimes children may not be able to fully express the root of their discomfort. Were there any specific incidents that stood out, such as a physical altercation with a friend, exclusion, or something else? If you saw something, addressing it directly from there would be more clarifying.
Children’s dynamics among themselves can change very quickly, but external interventions can disrupt these balances even more. Have you ever noticed how that friend interacts with other children in the classroom or within the group? Observing not only your child but also their general behaviors can provide some insight.
Don’t forget to look at your child’s attitude towards their other friends as well. There might be a general feeling of hesitation or they might be experiencing similar things in certain situations. To understand if the issue is limited to just this friend, a broader perspective is needed.
How does your child approach that friend? Is he keeping his distance completely or is he communicating cautiously? The way he reacts could be one of the things you need to observe. If the situation is completely distant, it might be a problem that he “knows” rather than one that he “feels.”
So, how does that friend interact with other adults besides your child? Is there any interaction you’ve observed with teachers, parents, or in another setting? Their approach to adults might provide some insight.
Even if your child has a clear reaction to that friend, have you observed how the other kids in the friend group approach this person? They may not be having an issue with just one person, but with the overall dynamics. Perhaps the trouble stems from the way they communicate with everyone.
What is that friend’s family environment like? Some behaviors in children might directly stem from the dynamics in that setting. Keep an eye and ear out over there as well 
One more thing: Did your child get along well with that friend before and then distance themselves? Or have they kept their distance from the beginning? If this change happened after an event, it’s important to try to understand the reason. If it has always been there, a more general assessment may be necessary.
If your child is uncomfortable with this friend but cannot quite express why, maybe they are sensing a threat instead of specific behaviors. Some children are quicker to pick up on subtle pressures or changes in demeanor from others. Do you think your child is someone who is sensitive to such things?
It’s also important whether families know each other or not. Maybe something that bothers you is seen as an explainable behavior elsewhere. Look for whether there is a connection in the background, not just on the surface.
Have you ever noticed how your friend behaves when he doesn’t have children around? If he shows a change in behavior in the presence of children or in other social environments, there might be something different going on there. Look not only at the one-on-one interactions but also at his general attitude.
Understanding children’s language is difficult, but sometimes you need to pay attention to what they say during playtime. They might be conveying something, even if it’s just a joke. Stay alert 
Does your child behave differently around this friend, maybe more reserved or shy? If there’s a situation where they don’t feel comfortable and it becomes clear in the presence of this friend, that could be a clue. Children’s energies sometimes repel each other; it doesn’t always have to be a major event.
Perhaps your child feels that spending time with this friend is an obligation. For example, due to something like being paired up by the teacher at school. Could it be that they are being distant because it was not their choice?
Is your child showing a direct sign of fear towards this friend? For example, avoiding physical contact or not wanting to go near them. If so, there could be something more concrete going on. Otherwise, the emotional distance might stem from a different dynamic.