A contract is a contract
If the contract didnt mentioned that it will disolve if one of the parties dies, the contract is still valid. Meaning that the videographer has to fullfil his side of the agreement (making a video about the weeding) and that the deposit is not refundable. The videographer offered to extend his side of the contract to a later date and therefore is still holding to the agreement. Therefore the remaining party of the couple has also hold to theirs side of the agreement, although it might be a little cynical to do the job on the next wedding of the remaining person. Additionally the non-refundable deposit is meant for expenses the videographer has to pay upfront, which he might also not get back, if the contract is disolved. Therefore he was asking for the non-refundable deposit.