Friend joining bulb...house move

Hi
I am currently with bulb myself and a friend of mine is completing on a property this week.
they want to use my refer a friend link however the property sellers have untilities warehouse. at what point does my friend need to join bulb? now or when moved in? in order for it to switch.
thanks
Linda

See: https://help.bulb.co.uk/hc/en-us/articles/115001245152-What-to-do-when-you-re-moving-house

Your friend will not be able to start a switch of the property until they have legal responsibility for it, which means not until after they complete. Once that’s done they should contact the existing supplier to arrange payment for usage from that date with their opening meter readings. Then they can start a supplier switch to Bulb (taking 3 weeks) using your referral code.

Thats great, many thanks for the help.