OP - you are assuming because of this downside of her choices, then they were the wrong choices overall.
It could well be she wasn't interested in evening classes or work or broadening her life because she was happy with it. Why change a life you are happy with now because in the future it would have to change? Your argument seems to be because it wasn't forever it was completely wrong at the time, but that's not automatically the case.
I have met many older men and woman who have had important and intense careers with no outside interests or hobbies, not wide group of close friends (more aquaintences or for many men, just their DW's friends DHs), then retired and found the change hard, working out how to fill their time.
Most do develop new hobbies and friendships, and i'm sure your SIL, once she's got over the end of one step in her life will be ready to do the next (and being younger, she's more likely to find it easier). It doesn't mean she was completely wrong in her choice for her family at the time to be a long term SAHM - plus if she had taken the choice to return to work, she doesn't sound like the sort of person who would have done lots of courses and had hobbies then either, so would have had the same issue upon retirement.
Some people aren't the 'hobbies' and 'wide range of friends' types. She's probably the 'pottering around the house' type.
She's a different sort of person to you. She might be depressed, or she might be a quiet person who doesn't want lots of "activities" going on.
Does she really not have any friends? Or just not ones you know about? What does she do all day when her DCs are out? at 18 it's unlikely they are taking up a lot of her time and energy, she'll miss them, but I would imagine her life will be pretty similar to it is now, if you are the sort to love spending time with your teenage DCs and be really be sad when your DCs leave home, it's unlikely that a few evening classes would stop you missing them.