Highest level.

There are many people who are already at the end of the map - some estimate more than a million people. For mobile devices, that is now level 18380 (although 45 more levels will probably be released in a few more hours - they add 45 new levels each week). So your mother in law is only about 1000 levels from the end, which is really good and something she can be proud of, but again, there are probably about a million players beyond that level. I am on level 17886 and am hoping to reach the end of the map in about 2 weeks from now to join all the others there. 😀

In case you feel discouraged that there might be a million users that high, keep in mind that Candy Crush Saga has over 180 million active users, and had over 360 million active users in its prime. So to be in the top million is actually pretty decent and puts your mother in law in the top 0.5% of players, at least in terms of number of levels passed.