I wouldn’t try to make his character more powerful, but give him alternative ways of helping.
There are some enemies that are inherently harder for some classes or characters to fight. That doesn’t mean that they’re weak, or can’t assist during a fight; it just means they shouldn’t be on the front lines(usually.)
A good example is creatures with magic resistance, which makes them very potent against spell casters. That doesn’t mean you should make the spell casters stronger, or “more helpful,” as it will end up making them even more powerful when this weakness isn’t highlighted.
Perhaps the monk could body-block for one of his party members and take the Dodge action, preventing the Balor from advancing toward his allies. Effectively, he should be playing defensively, as the majority of damage is triggered by attacking the balor. This is more-or-less the intended line of play. It should be a challenge for a player to overcome.
Personally, if my player can sell me on something, I’ll allow it. I actually had a fighter that doused himself with water before running into melee against a balor. I loved the idea, so I gave him resistance to fire damage for that turn, and he waylaid the balor and then fell back on his next turn.
I think good ideas should be rewarded, as it encourages outside-the-box thinking and leads to more fun moments in combat, and usually, makes the player feel like they were contributing to the encounter. When fighting deadly enemies such as this, some strategy is expected and even require.
A monk charging headlong into combat against a balor and continually soaking up damage is less of a strategy, and more of an action. Rather than simply give the character an item to resist the damage or otherwise counter the “rebound damage“, give them alternative ways of contributing to the encounter.
But, that’s just my 2 cents.