I don't know how you found out how much the other offer was for, but if her boyfriend is really ready to buy the house, either you go higher, or he buys the house - the ex-husband should sign whichever offer nets them the most money. If that is from the new boyfriend, so be it.
The "first offer" has no rights to it. The first accepted offer does - that means accepted by all parties that need to sign. Until there is an agreement (by all), there is no agreement at all.