family visitor - return ticket

Hi,

My father has recently been issued family visitor visa and will be flying soon.
Should i book one way or return ticket for him?. I am not sure of his return date.
Having one way ticket will create any issue at immigration control either in the UK or India?

Thanks,
Suresh

Always best to have a return ticket as if he shows up with one way ticket, he will be questioned why and may be declined entry if he couldn't satisfy.
Better be safe than sorry...