The mechanism she's asking for is a performance obligation, usually a deadline clause in the purchase agreement or a side agreement, backed by a recorded instrument in her favor so it has teeth. That's typically a performance deed of trust or mortgage securing your obligation to refinance or sell by a date, with a remedy if you don't. Drafting and enforceability there depend on state law and it needs an attorney in the property's state, and you should know that a recorded performance instrument sits in your chain of title and has to be released before any new lender funds you.
What no clause can do is make a refinance available. Your own numbers show it. Raise the payment by roughly $550 a month and the property has to carry it, and if rents don't, no DSCR lender writes the loan regardless of what you signed. A deadline you can't meet converts into her remedy, which means either you deed it back or she forecloses on the performance instrument. Price that outcome before you agree to the date.
There's a better path for her problem that people skip. FHA loans are generally assumable, with the servicer qualifying the new borrower and occupancy rules applying, and a completed assumption releases her from liability, which is what she actually wants. It's slow, it costs fees, and an investor may not clear the occupancy requirement, so confirm the specific terms with the servicer in writing before dismissing it.
The other thing to hand her is the underwriting question she hasn't asked her own loan officer. Some underwriters will offset a departing property's payment with documented lease income and a payment history, so her ratio problem may be smaller than she's been told. That's a question for her lender, answered in writing, not something you should be solving with a 24-month promise.
