Is it a conflict of interest when the law guardian has known the opposing sides key witness for 3 years?

Probably not, depending on the nature and extent of their relationship. It is very common in the legal arena to have people involved who know each other. The lawyers know the judges and professionals who serve as witnesses (counselors, accountants, etc.), the lawyers know each other, the professionals know each other, etc. This is why it is important to have a lawyer, so you don't feel like an outsider.