notgran
I would totally do it, it's his Mum for goodness sake and 40 miles is less of a distance than my daughter commutes each day. I would also go with them and have a lovely day out spending the money you aren't spending on gifts/drinks etc at the wedding. To use that overworked phrase, Be Kind (to your MiL)
Absolutely this.
There are often threads on here that suggest that people put invitations to weddings ahead of their feelings for their families, which I see as odd.
My sister has four children, all of whom have had weddings. One was abroad (I was invited but didn't go) two were in the UK - I was invited to one but not the other, and the fourth was nearby, but a small affair, which with a large family like my sister's means 'immediate family only'. I babysat for the children who were too young to sit through the reception - Mr Dog went to the venue to pick them up and brought them here for the afternoon. All the couples got the same present (a cheque for the same amount). Whether or not they could afford to (or want to) have a large wedding with lots of guests has absolutely nothing to do with how I feel about any of them.
My son got married recently and had a small family wedding and a large party for wider family and friends later. The bride is an introvert and didn't want a big wedding with all eyes on her, which is absolutely fine, surely?
My MIL was unable to come because of the journey, but my SIL (her daughter) offered to drive her about 100 miles to the venue, find something to do in the surrounding area for a few hours and bring her back so she didn't need to stay in a hotel. SIL wasn't invited, but wanted to give her mum the chance to see her grandson get married. It was kind of her, but very normal, I think? As Notgran says, a 40 mile round trip is the sort of distance people travel to work, not an overland trek.