I know at least one is getting frustrated with combat because he can’t roll to save his life.

Yeah, that’s a feature of 5e combat, not a bug. It’s what makes me despise combat. I miss three times, wait 20 minutes for my turn to come back around, miss three more times, wait 18 minutes, and then combat is over.

Some of us are just cursed. The only workarounds I’ve found so far are:

1. Specialize in making the DM roll saving throws, rather than me rolling attack rolls. A spellcaster who focuses on save-for-half spells feels so much better (because even when the monsters pass the save, the player still get to feel useful).

2. Specialize in party buffs and reaction spells. They don’t have to roll anything to Enlarge or Dragon’s Breath their friends, and they get to feel like they helped. Also, never underestimate how good it can feel to make a Counterspell bot. Even if the bad guys start upcasting their spells and your player always fails the check, they still made them waste a higher-level spell slot than they’d have used otherwise.

3. Halfling Divination Wizard with the Lucky feat. Three re-rolls, two portent dice, and rerolling all 1s once really helps brute force one’s way through being cursed. And it’s not broken when people like us play it, because we end up finally managing to get around the same number of successes that non-cursed people get normally.

Notice that none of these solutions are possible with pure martial classes. Steer your player away from those, maybe even let him make a new character. Martials are totally at the mercy of the dice.

My ultimate solution was to switch systems and play FATE instead. But that’s an extreme reaction to an extreme level of frustration.