Legal Question in Real Estate Law in California

There could be a strong argument that ownership doesn't divide according to what's shown on legal (recorded) title, but equitably should, can and will divide according to the parties' relative contributions to the down-payment. So, as an additional factor, I'd need to know what either of them can show, by clear and convincing evidence, as to how much each of them contributed to the purchase money (down-payment). If he can show that he provided 90% of the down-payment, at or before the closing, he could argue for a 90% ownership share. Such a claim could be nullified by evidence that he intended to make a gift of the excess (above 50%) contribution.

Then, if and when the property is sold, or she buys out his interest, a court having jurisdiction following the filing of a "partition" suit could make additional adjustments to the net proceeds each receives. Such adjustments would be calculated to reimburse one or the other of them for excess outlays (in excess of his/her share of ownership) for necessary expenses such as mortgage interest, property taxes, insurance, and necessary repairs.

If the couple is fairly close to agreement on percentages (40/60 vs. 45/55), a lawsuit may be undesirable. If, on the other hand, there is substantial equity in the property and the down-payment made by one is well outside the ranges being negotiated, it may be worthwhile for that person to litigate under "purchase-money resulting trust" and "partition" principles.

Negotiate a deal he can agree to, or litigate. No mystery here.