Spotlights and deposit disputes

This seems exorbitant to me.

Quite.

I'm pretty sure agents can only charge for the cost of the bulb.

Are you? Who fits them for the new Tenants? Who goes out to buy bulbs that are matching in type and colour temperature and wattage? No-one wants mismatching bulbs, do they?

...like they can take the money without her agreement, no breakdown of costs etc.

They cannot. But the question that arises (as it always must) is why didn't your girlfriend just replace them? She knew it was her responsibility. She had plenty of opportunity (and it's unlikely all 3 bulbs went just before the day she moved out. So, is she just lazy? Does she just assume that people will clean up after her? What is it that makes a person behave this way? What do you think? You say that a Check-Out found 3 deceased bulbs... well, not really... right. She wasn't unaware, right? Does she use the bathroom without lights generally?

I have suggested she dispute the deposit deduction with the independent arbiter.

Very likely to get a good outcome here... but an even better approach might be for her (or is it you who will do it for her, as she doesn't quite seem on-the-ball when looking after her own interests) to write to the Agent and suggest that this be assessed by the Deposit Scheme's Alternative Dispute Resolution process... and then - at the same time - offer an olive branch whereby, say, she offers the Agency an amount that will tempt them (for an easy life for both parties, you understand?)... maybe something like £5 per bulb and £35 for sourcing and fitting? All-in... £50. This could be a short-cut, because actually going through with a Deposit dispute is likely to yield good results, but will take time (and some effort)... and who wants that hanging over them when you can just move on... and groove on.

Faced with £50 in the hand, or possibly £150 (but likely less, probably not zilch) in the future I reckon the Agency might jump at it.

£50 is still a lot for 3 bulbs. Sure. But doesn't your girlfriend probably need to learn something at the same time? No better way to learn than through the purse.

Maybe next time she'll actually be bothered to try and hand the property back in the same state it was when she took on the tenancy.

It seems a lot to me.  If the agent or landlord has their tenancy agreement written correctly they can certainly charge a call-out fee plus the cost of the bulbs.

I used to work as a property manager in London. Our contractors call-out fee was £78 then they charged for the bulbs.

They might be charging a bit too much but why didn't you just change your own light bulbs and expect someone to go to the time and effort to do such a simple task for you?