Is there any legally acceptable way he can dispose of his tenant's car ?
Yes, probably.
If the tenancy is still extant and rent is owed then the landlord can levy distress - that is, seize the tenant's property in the gararge and, if the tenant still won't pay up, sell it to clear the debt - any excess after rent and costs has to be given back to the tenant. Distress is still lawful, though probably not for much longer. There are a few tricks and traps mind, so do some research - I'm not advising you to do it, just giving you my unqualified opinion that I believe it's possible.
If the tenancy has ended then if the car has been abandoned then the landlord can sell it without any more to-do.
However, just because the car was left behind doesn't in itself mean it was abandoned, and if the tenant just left it because she couldn't be asked to make arrangements to move it then it's actually still the tenant's and the landlord has a duty to look after it - the landlord is an involuntary bailee - see the Torts (Interference with Goods) Act 1977.
The landlord really needs to terminate the tenancy if the tenant is no longer in a position to pay the rent.