His MIL is definitely handicapped as she needs special assistance getting on the plane, and she can ride in a wheelchair just as easily as walking. The OP can push her to the gate and that's all there is to it. Making a big deal out of somebody that truly needs special handling when half of the people that pre-board are faking it is simi-ridiculous! At most airports the OP can request a wheelchair and a gate pass and take her to the gate himself, so do it! This is a once in a life time experience and the OP is very concerned, so do the card thing for the f/a's, give her a note for the other end, and take her to the plane.