RAW is pretty well covered here, but here are some options you might consider as a DM:
* Tell the player that under RAW they do not get advantage, but you feel like that robs the player a bit. Accordingly, you'll let them roll a religion roll to try to gain advantage. This involves an extra roll before they can make the attack roll, but it lets them feel like you gave them a chance. In my experience, most players decline this extra roll, taking the RAW explanation, but they feel like you gave them an opportunity.
* Tell the player that under RAW it is not allowed, but if they can give a description to you why they should get the advantage you'll allow it. This encourages the players to 'get into' the game more, increasing immersion and adding to the fun. Again, players will decline, but in my experience they're more likely to do this than take the extra religion roll.
* Let them have it and say nothing about the RAW situation. I often let the RAW fall by the wayside in favor of a more fun ruling for the players. I'm not concerned when my players succeed - and l enjoy it when they revel in their great successes.
There was a time not too long ago when a PC with an upcast Spiritiual weapon (2d8+5) had the weapon just out of reach of the BBEG (it had been cleaning up fodder). The PC had cast a banishment spell, and the BBEG made the save. The BBEG was going to go next, and it was going to unleash a massive area attack that would likely have taken down multiple PCs. The PC moved the spiritual weapon next to the BBEG and was about to roll an attack when I pointed out that they'd moved it too far. They recounted it and their face dropped when they realized I was right. I gave them a chance to push it further by making a DC 20 religion roll. They grabbed the dice, made the roll and rolled well. Then, they rolled a critical hit, followed by a high damage roll, and managed to finish the foe off by doing ~30 damage. It was a great moment in the game that would not have taken place without giving the PCs a chance to exceed the RAW by just a bit.