What ram do I get for boyfriends computer

My boyfriend has a computer he got when he got in 2017 and ive recently heard him complaining about needing more RAM but it being a super weird and random place to get it and that it was pretty pricey. He's not really the easiest to shop for but I thought for our 1 year I could look into getting him some so his computer can run a little better !!

TDWR
im looking for RAM for a... HP Omen 880-026na Core i7-7700 16GB 2TB + 256GB SSD GeForce GTX 1080 8GB DVD-RW Windows 10 Home Gaming Desktop

thats really on the info I have on his computer Im pretty technically challenged so please let me know how and where to buy and if this is even possible!