How late do you have to be on your mortgage payments before a bank will start foreclosure procedings?

My sister, a nurse, is off work due to a broken leg and her husband (deadbeat) quit his job just a couple of days after she got hurt and hasn't gotten a new one in the last 5 weeks. She doesn't have sick leave or temporary disability insurance. She missed this month's mortgage payment. She will probably be off work another 4-6 weeks if her leg heals okay. So that means missing at least a second and maybe a third payment. But her doctor worries with the type of fracture she had that it may rebreak after she gets the cast off and tries to walk on it with even a boot. Then he would have to do surgery with plates and pins. She could be off work for up to 3 months more if that happens, which would put her missing almost 6 payments. I don't want her to lose her house, but I don't want to give her any money until she is truly at risk because I don't want her husband to benefit from it.