A tenancy agreement is a binding contract. So your Son and his girlfriend are bound by it. A contract can be breached like any other - whether a tenancy contract can be terminated depends on the facts and if the breach on the landlord's part is serious or not. Being misled can make a contract void too. It sounds like they were misled in that they were told the flat was in great condition - so there is a case if they wanted to terminate and leave. However don't be surprised if the landlord and agency kick up a fuss and try to argue otherwise.
In UK law, landlords must ensure the property is in good repair under Section 11 to 16 of the Landlord and Tenant Act 1985. Further, a recent law came in to force to provide more protection to tenants, which is the Homes (Fitness for Human Habitation) Act 2018 is now in force - the house or flat needs to be healthy, safe and fit for human habitation (free from things that could cause serious harm). So damp and an untreated leak would put the landlord in breach of this law.
If they do not, they face prosecution by the local council and can be fined. It would also be a breach of contract on their part.
*The only situations where a tenancy can be ended are, you finish the tenancy term or there is a break clause meaning you can end the tenancy part way through. Or the landlord agrees to you leaving early - usually if another tenant can be found. You could terminate but you would need evidence of repeated requests to the landlord to fix and they do nothing. If you terminated then they could sue and you may have to defend the claim in a civil court*.
The Local Authority Environmental Health Officers have powers under the Housing Act 2004. They have a duty to ensure that properties in their area are in a habitable condition, and will serve improvement notices on landlords of properties which are assessed, under the new Housing Health and Safety Rating System as having ‘category 1 hazards’.
Similar powers are available to Local Authorities under the Environmental Protection Act 1990 if the property is considered a threat to public health.
I would therefore recommend that you contact your local council (the housing department) to report the condition of the property. They may be able to help further with this - it should put pressure on the landlord to agree to the termination. A law firm may have more of an effect too, especially if the landlord takes legal action. You could come back to this site as an alternative though - feel free to come back to the site in future if you need more help, or if you need recommendations for suitable law firms.