I’ve been putting together a lot of old laptops for friends and family, so here’s my opinion based on my experience:
Your CPU speed is ok (that CPU scores 932 passmark points, which is ok for dekstop usage and youtube at 360p (you should set up his youtube at 360p, and to not autoplay). If this was Chrome, that cpu could do 480p, but firefox is much slower than chrome on youtube. The difference in speed is not visible on fast systems, but it is visible on very old ones (anything less than 1500 passmark points).
Your biggest problem is the RAM. You have only 2.77 GB of RAM, which is NOT enough for normal web browsing in this day and age – if you’re using lots of tabs. The moment you will open more than 2-3 tabs with heavy websites (e.g. facebook, nytimes, and linkedin), you will start swapping like crazy with Cinnamon. So your user will always have to be conscious of what apps they have open (and make sure you configure 4 GB of swap too, just in case).
Mate and XFCE should be using less RAM, indeed (about 600-800 MB instead of 1.3 GB on Cinnamon). I find XFCE more stable personally, and it only uses 100 MB more RAM than Mate on average. The only good thing Mate has over XFCE is that it comes with a user administrative gui app. I usually install that on xfce (“mate-user-admin”).