Re: What happens when a mortgage is called
Nearly all "mortgages" (actually, pairings of a promissory note with a deed of trust) contain a "due-on-sale" clause. "Due on sale" means that the entire loan principal balance becomes immediately due and payable when the original owner-borrower sells the collateral (the house).
The solution, which avoids at least one of your brother-in-law's problems (i.e., foreclosure) is to pay off the existing loan, either with cash or by refinancing.
In order to refinance, ownership of the house must be clear and unclouded; and, of course, only the true owner can refinance. Sounds as though the girlfriend is the owner of record, and she is the one who will have to do the re-fi.
Assuming the transfer of the house wasn't done to try to put it out of reach of his creditors or hide assets, or the like, and assuming the two of them can still cooperate on business matters, the two of them should work together to get the old loan paid off before the lender initiates foreclosure proceedings.
Unfortunately, "signing over" a house without getting any money for it, or getting too little, smells of fraud, especially when the donee is a relative or friend. If your brother-in-law has creditors or is facing a judgment, the creditors will be onto this house transfer like flies on a barnyard. House sales or gifts leave a distinct factual trail (at the recorder's office, the assessor's office, and the tax collector's office) that is easily investigated. If there is a discoverable trace of fraud, refinancing may be impossible and a creditor suit attacking both of them is likely.
On the other hand, if there was no equity in the house, there's no fraud, because the creditors were deprived of nothing.