I have a litter of 5 week old puppies. Their mum has decided to wean them and they are finally doing very well on solids. However they have now found out that they can suckle from another of my bitches. This bitch has never  had a litter and is not producing any milk however she just stands there and lets the puppies suckle from her. Is there anything I can do to stop them apart from keeping them separate? Will it do her any harm?

You do need to stop this suckling of the other bitch.
The puppies suckling her especially with so much co operation and pleasure from her will bring her into milk. ( there is also the posibility of it causing mastitis )
Having weaned your puppies you do not want another female feeding them!   Keep them apart for everyones benefit.
Best wishes.