Client hasn’t filed individual taxes since 2019 and has no copies of prior year partnership returns. How would you proceed?

Prospective client reached out asking if I could help her get caught up with taxes and I agreed. She uploaded her 1040 from 2019 (the most recently filed 1040), and it turns out she has a partnership. The partnership had a large loss in 2019. She thinks the other partner is her dad or brother. Apparently her prior accountant set it up for her business ventures.

The headache is her prior accountant went out of business. She can’t get in touch with them and doesn’t have copies of her partnership returns. I assume those returns haven’t been filed since 2019 either. Not sure if she’s been getting notices of not filing, she hasn’t mentioned.

I’m not sure how to begin helping her without a copy of the most recently filed partnership return. I don’t even know what her ownership percentage is. It doesn’t seem the other partner has copies of anything either. Anyone have advice on how to proceed? Maybe a POA and calling the IRS? This is the first time the situation has come up.