The fact they are printed TWO PART RETURN indicates quite strongly and irrefutably that they are two pieces of the same item. They are not two tickets - in fact the Return portion of tickets have easements that would mean to use the return portion in isolation is clearly fare evasion - for example, an off peak return Cardiff to Paddington is valid to return from Paddington after 0800. The first off peak train you could take on an off peak return or an off peak single is 0915, post peak. By using some one else's return portion you are evading the correct anytime fare travelling on that train. The fact it was bought for use by a company is irrelevant
Ignoring it being someone else's ticket for now, that is (legal) avoidance, not (criminal) evasion. There is absolutely no rule which bars throwing away the outward portion and only using the return portion (or vice versa, provided the throwing away of the return portion occurs after the completion of the outward journey), even where that saves the passenger money.
The only thing you cannot do in that context is to use the return portion AND THEN use the outward portion. Use of the return portion of one given ticket invalidates the outward portion.
This falls into the same bracket as splitting, or using a longer ticket for a shorter journey where BoJ is permitted. The TOCs might not like it but it is totally legal.
There are many ways to do this kind of thing. A common one where there are asymmetric peak restrictions is for a weekly commuter to start their Monday-Friday commute with a single, then purchase returns from the destination back home, then finish with a single. No rule at all against doing that, and I imagine it is very common practice.