The fares do exist in the system for London Eurostar CIV to Spital,
according to BRFares - I'm not sure whether you can get any booking engines to offer them. It would have been very amusing to do a claim under CIV regulations to Merseyrail.
If they don't want to fork out for a taxi, there is the
number 1 bus from Liverpool to Chester. I don't think it goes into Spital village itself, however they could alight on Spital Road and then take the
84/85 service (if the timings work - it's a daytime-only service). BUT cross-river services will depart from Pall Mall until 1300 on the Saturday and Sir Thomas Street from 1300-1830, not serving any other stops in Liverpool, so there would still be quite a bit of walking involved. Integrated transport, eh?