On the purchase of any house whether is it a right-to-buy, or from a private owner, the property will always be made subject to any existing restrictions or easements. Easement is a legal term for rights in favour of third parties, such as a right of way, or a right to connect into drains across the land.
At the time of your client’s purchase, any right of access the council wished to retain would have (or should have) been disclosed. The first thing to do is for your client’s solicitor to check with the deeds whether this particular easement is registered against the property. They may need to check with the Land Registry or, if the property is not yet registered, in the title deeds themselves. If the easement is enforceable, then the property has to be sold with that right imposed.
Obviously the buyers will need to consider how that third-party right affects their enjoyment of the property. If it is across one small portion of a large garden it may not. If it across a large drive it may not, but I suspect the mere existence of such a right is going to reduce the buyer’s enjoyment of the property. Your client must accept the value of their property is likely to be reduced by the very existence of the right ‘ how much will be down to the buyer’s valuer.
There could potentially be a claim against the solicitors acting for the clients when they bought the property, if they did not disclose any pre-existing right of access. But any consideration of action against those solicitors would require a detailed examination of the papers at the time of purchase and what was said or not said to the sellers.