If my ex has decided to buy a house with her boyfriend and she defaults on the mortgage, am I financially responsible for her debt?

Unfortunately, you live in California, a community property debt. While technically *you* are not liable personally for the debt, your share of "community" property, which is all money and assets acquired during marriage other than anything willed or gifted to you, is liable for the debt. Your separate property--anything inherited, gifted, or acquired pre-marriage--is not liable.
Example--deliberately simplied:
Say your wife and her boyfriend take out a $500k mortgage which still stands at $480k in principal when they default. Say that the house brings in $380k in foreclosure, leaving $100k unpaid.
Now, say that during your marriage, you and your wife bought a house  together; bought two cars; and put $25k of income from work one or both of you did during marriage into a joint bank account or accounts. (And remember: you are continuing to earn money while you are still married to her, and your new earnings during marriage also become community property.) The bank/lender who is owed the remaining $100k from the defaulted house can go after the $25k in the bank, the cars, or your home (though any existing mortgages or financing agreements on cars or house take precedence and could effectively make those assets not reachable by the lender, meaning that the money in the bank may be the main thing they go after). That's because those assets were acquired during marriage, so they are community property; and community property can be used to satisfy the debts of *either* spouse which he or she incurs during marriage--even while estranged. You may wish to accelerate you divorce to disentangle yourself from her--not only can she obligate your community property to be used to pay her mortgage, but suppose she runs up big credit card bills while still married to you? Co-signs for an expensive car for her boyfriend, which he defaults on? Incurs large medical debts? So long as you are married, even if estragned, your community property is potentially responsible for these things.