That is a ridiculous amount to be charged to get the plugs out (and replace them), regardless of how long it took them. Once the garage could not easily remove them – they should have advised him of the inflated costs that will be charged as a result, and ask if they should proceed.
For that money you would get at least 4 hours mechanics' time. Sounds like he’s been charged for the mechanic to hang around while the plugs soak.
He might get a reduction if he complains (or takes the matter up with local trading standards) – but don’t use the garage again, and advise others of his experience with this garage.