N.W.
Chances are your ex-friend's kid is asking the same questions!
Do you have this lady's email? Is there any way you could send her an email telling her that you understand that you have differences but your son really likes his son so for their sake could you set your differences aside and let them play? I only say email because you can write a well-thought out email and it's not confrontational.
Another idea is to have your son call up his friend. 5 years old is old enough to talk on the phone. You can pick some times for a playdate and be right there to coach him through the conversation. That way you don't have to talk to his mom. Maybe you could have the kids meet at a neutral place.
If you think that would bomb I would go for the truth. Tell him you and his mommy had a disagreement and now it's hard for you to ask for a playdate. Assure him his friend still likes him, maybe have him send a letter so they can be pen-pals.
What a hard situation. I would hope the other mom can come around sometime!