Ok so a mage casts passenger on the street sam takes the drain and uses a sustaining focus to handle it from there.

street sam walks round the corner (but still within spell range) can the mage use the sam's borrowed eyes to target someone?

Logic says yes but it sounds abit ultra powerful

Since Borrow Sense - whch is the basic, single-sense spell Passenger is based on is explicitely stated as not allowing to use the spell's target location/senses for spellcasting, I'd say "no".

Clairvoyance also specifically states that you cannot use its LoS to cast with, and both spells are based on that as well. Basically if it isn't the mage seeing it, it doesn't count.

I take it this spell is from Digital Grimoire?