Let's say that your FBIL (also FI's best man) has a girlfriend. Girlfriend has an extremely rocky relationship with FMIL. Girlfriend has even gone so far as to threaten FMIL. However, girlfriend lives with FBIL and she helps take care of FFIL, who is ill. (Oh, FMIL and FFIL are divorced and have been for 20 years).
FBIL told both FMIL and FI that he does not want to bring girlfriend with him to the wedding. He does not in any way want to attend with her. And I'm assuming that girlfriend does NOT know this.
Would you invite just him? Invite the girlfriend anyway? Invite him with a guest?