Ask your client how to proceed IMO ,
As for your own decision wrt HBU, I would look at the zoning - is it legal to have a four-unit improvement on the lot ? If the four units are legal, but two were built with no permit, Fannie and Freddie allow non-permitted dwellings, idk about for small income properties. What is the construction like, is it on sewer etc? Odd that they did not get permits
What kind of HC makes sense in a situation like this? If you are going to use an HC, then it should be subject to that HC ( not as is ). ask the client about that as well.