I suppose it makes no difference in the scheme of things, and I am sure that the operator will be forthcoming with a positive outcome, and I in no way wish to question somebody's disability, but:
1) Can't use a self service machine because of hygiene issues - OK
2) Potentially can't use a help point for advice/permission to travel because of hygiene issues - OK
3) Doesn't wish to use their own mobile device - fair enough, but would seem a reasonable compromise - but OK
4) Pays at the destination. Now if points 1 and 2 are accurate, you would have to assume that the customer can't pay by debit or credit card either, (same hygiene issues with pressing buttons/handling a public chip and pin device), and even more unlikely to be using cash (!!) unless they have sterilised it and cleaned it before handling it....
All somewhat moot, because the operator will be unlikely to question the complaint, but I would be interested how the original poster eventually purchased the (more expensive) ticket. It's easy to blame the rail operator, but the operator is obliged to only take reasonable steps, not every step possible.