Is it legal if my boyfriend’s ex-wife just used my past records almost 10 years ago to get sole custody of their child?

If you have ten years of good experience being around the child, it would be very hard for the mother to make a strong case for reducing placement of the child with the child's father. How can she make the claim? It's a free country but that does not mean of itself that every allegation made in court papers is true. If she should start a proceeding to change custody, find a skilled matrimonial lawyer in your locality. It's almost always worth the investment.