The MIL unit presumably within the same lot is a separate structure.  What you haven't stated is whether the MIL unit was used exclusively for AirBnB or whether your client actually did use and occupy that unit as part of his principal residence pursuant to Treas. Reg. §1.121-1(b) and (c).

The MIL unit being a separate structure, whether §121 may apply and, if so, to what extent would hinge on whether the use (and inevitable the occupancy) requirement, which is determined based on facts and circumstances, was satisfied as per Treas. Reg. §1.121-1(e)(1).