Who pays?

[Eatcabbage]

Asking for a friend.
Friend's ex had an affair and then sometime (years) later decided to move out of the family home. Both were against friend's wishes.
More than 5 years on, ex has done nothing in the way of discussing or instigating divorce, despite having bought their own house and being in another (not the affair) relationship.
If friend now wants to move on and get divorced, who pays for the divorce if friend starts proceedings? (Ex is apparently surprised by the suggestion.)
Thanks.

Whoever files petition will have to pay the court fee (£550?), but there is a box I think on the form where you can tick if you want to recover costs in some way. The petition form, and guidance are on the gov website.