When the biological parents live together, unmarried, it creates a different type of Tax client/s.

Sounds like they have been DIY's in the past and may be coming to a preparer this year because they are afraid losing $$ because they don't understand how it's suppose to work for 2020 returns. 

You may want to get a "Conflict of Interest Waiver" from each of them. Then if they separate, unhappily unmarried, you will have provided that vital CYA measure for yourself.