Playing D&D should be fun. I highly suspect that a player made to feel like they're ruining everyone else's fun because they didn't study their character hard enough is probably a player that's not having fun.
Character sheet apps are a mixed bag. It's great because they automatically calculate proficiencies, saves, skill check bonuses, attack bonuses, etc. for you. They also (particularly D&D Beyond) are flipping BRILLIANT at spell management, special ability management, and wildshape management IF you have access on DDB to all the sourcebooks you need.
They can be a challenge - especially on the phone - because of limited screen size. If you go that route, encourage a tablet or laptop because it's easier to navigate. Otherwise, encourage pen and paper (or better - paper sheet inside a plastic sheet protector with a dry-erase marker) and go over it with them to make sure it's calculated correctly. Then for spells and such, make them a list or cards. For special conditions like hunter's mark, hex, etc., make cards they can turn over when those things are in play that show the bonus they need to add. (Also yes, help them do this stuff. Don't do it all FOR them, but at least show them what you mean and get them started.)
Character sheet apps are a mixed bag. It's great because they automatically calculate proficiencies, saves, skill check bonuses, attack bonuses, etc. for you. They also (particularly D&D Beyond) are flipping BRILLIANT at spell management, special ability management, and wildshape management IF you have access on DDB to all the sourcebooks you need.
They can be a challenge - especially on the phone - because of limited screen size. If you go that route, encourage a tablet or laptop because it's easier to navigate. Otherwise, encourage pen and paper (or better - paper sheet inside a plastic sheet protector with a dry-erase marker) and go over it with them to make sure it's calculated correctly. Then for spells and such, make them a list or cards. For special conditions like hunter's mark, hex, etc., make cards they can turn over when those things are in play that show the bonus they need to add. (Also yes, help them do this stuff. Don't do it all FOR them, but at least show them what you mean and get them started.)