Help for a friend please

[hermesgrrrr]

Friend has been separated from her ex for a number of years, he has been in touch to say he wants a divorce (wants to remarry) and she has to pay £550, according to "the solicitor" he has seen. No children/assets together so very simple process.

Logic tells me that if he wants a divorce, he pays, right?

Right. £550 are the court fees for a divorce petition. As he's more desperate than her, he should pay & raise the petition.

yes it totally agree,

£550 sounds about right for the Court fee, these days. Yes the person who 'petitions' (i.e. applies) has to pay the fee -- though in amicable couples, it is not unknown to split the fee.
Your friend could threaten to prolong the process if she's feeling difficult -- people often expect that they can remarry quite quickly when the process can take quite a while, of course.
If grounds are separation after 5 years, only his permission needed. If 2-5 year then both need to agree; or else mention some unreasonable behaviour.