It sounds like your mother-in-law is overly dependent on your husband. This is a very, very tough situation to change. The options for change can depend on what financial resources your mother-in-law has. If she can afford to hire someone to mow her lawn, I think that would make more sense than your husband traveling 40 miles to do the job. (It might even be worthwhile for your husband to pay someone, on her behalf, to do the lawn, rather than spend the time and gas-money it takes to travel 40 miles and back.) If he flat out refuses to encourage her to do that, then you really are up against a wall.

I don't think it's really a case of your husband putting his mother first, so much as it is a case of him trying too hard to maintain her household, as well as his own. Unfortunately, his mom is going to keep getting older and even more dependent on him. She probably groomed her children all their lives to be overly involved with her. Most likely, she will never willingly change. Try to understand how hard it is for him to change the dynamics of this.

His mom has him programmed to feel guilty, if he is not looking after all sorts of things that she probably could make her own arrangements for. He just doesn't want to feel guilty. He thinks he is being a "stand-up guy" by taking on all that she puts on him. You have to approach this with care. He is under a strain feeling that he can't let her down. If it really seems that his mom is exploiting his loyalty to her, then you might need to explain that to him in a gentle, but firm, way. Try to reassure him that his mom can be alright depending more on her own resources. This can take a lot of patience and time and ingenuity, on your part, trying to re-program his thinking.

I don't think you are going to get much success by adding to his burden of guilt, by saying that he is neglecting you and his own household to go running 40 miles away all the time. Maybe you could suggest some trade-offs. Like, maybe, he could find someone locally to do her lawn, and still commit that you and he will go up on a regular basis to take her out to dinner, or spend an afternoon visiting. Mothers who do this to their children can be geniuses at keeping this pattern going. You'll have to be pretty darn smart, yourself, to figure a way to coax him into a new pattern. Show real concern for his mom's legitimate need to have contact with her son, while helping him to see how she could be more self-reliant. If he thinks you just resent her, then he will ignore your suggestions.