Prepaid only
This is credit for a prepaid line. Someone on a monthly contract is billed by direct debit and generally cannot apply top-up credit to that bill.
Check which they are on before asking. If it is a contract, this is the wrong category entirely and nothing here will help.
Bundles beat pay-as-you-go
Once the credit is on the account, the recipient chooses how to spend it. Buying a monthly bundle of minutes and data is almost always far better value than letting it drain at standard rates.
Say so when you hand it over. Plenty of prepaid users never switch to bundles and pay several times more than they need to.
One network, one country
Credit is issued per network and cannot be moved to another. It is also national — the same brand in another country is a different company.
There is no salvage if either is wrong. Ask; it is not a rude question.
Why it is a good gift
It is spent within days, entirely, on something needed. There is no remainder problem, no sizing risk, and no question of taste. For students, older relatives and anyone on a tight budget it lands better than most things costing twice as much.
It is also one of the few gifts where the recipient's reaction is relief rather than politeness.
Amount
Match a whole bundle. An amount covering one, three or six months of their usual bundle is clean; an amount covering two and a bit leaves a remainder that drains at expensive standard rates.
Asking for one
Confirm the network, the country, and that the line is prepaid. See the mobile top-up guide.