Since being dragged through court here is what I have learned.
1. Bm's lie so record everything telephone calls, meetings, any words spoken between parents.
2. Save all text, emails, and have a backup of everything because computers break and cell phones delete things randomly because shit happens
3. Get receipts for everything. If you pay bm for anything that's in her name get her to write out a receipt. Only pay in cheques.

Even if things are going great if dh does anything that bm does not like (such as having a new baby with wife) bms will cause trouble so my advice is to be prepared.