Firstly, get all your ducks in a row, admin, calls, emails, letters, photos (of the damage) and anything else you may have. Record them in chronological order. You may or will need them as evidence.
Check the deposit and who’s name its in.
Write to the tenant who left reminding them they are still liable for the rent, damage, repairs, upkeep, and any unpaid rental debt, sent it via first class and copy via email if you have it.
Also write to the remaining tenant clearly stating their position. Be careful they do not try to just pay half the rent stating the other person should be pursued.
Ask the remaining person to get in touch with the local benefits office to see what financial help can be given. The benefits office would rather keep someone at a property than have them in more expensive temporary accommodation. If the benefits office try to help by additionally trying to drive down the rent, you know what to say. They may try to pay some or all of the rent direct and if so make sure that it is on the condition that if the tenant is over paid or has issues with the benefits office that the recall of these funds will not be with you (as the tenant will not have money to repay it).
As Colin states change the locks and have your own keys, most AST state these may not be changed without consent. Add the cost of this to the tenancy , with receipts *(even if you have to ultimately get redress and take it out of the deposit). Get the tenant to sign for the new SINGULAR key set (all external doors and garage).
If you can visit (Covid conditions applied with) and chat with the tenant face to face to find a way forward it would help in more ways than one (the courts may ask you if you have and if not, why not), record with notice to the other parties called or if not take notes of the conversation. Also if the tenant is aware that in a couple of months a court order will kick in, their credit rating and ability to rent in the future will affect not only the next rental but be on their record for many years to come; they may do something to address the situation. Of course this has to be done in a professional non threatening manner.
If you are able to visit the property, tenant doesn’t need to be there, do an inspection and take photos and videos of the property and any damage. Any repairs, damage/replacements and costs should be notified immediately to the both tenants. Just because the other tenant did a runner will still not obviate their responsibility.