I have an old bathroom mirror protecting a bit wallpaper in my piggy roaming area.
The mirror will hold attention shortly, but not in the longer term because unlike with us humans, sight is not the strongest sense in guinea pigs; smell, hearing and touch are more important for them. That kind of multi-sensory interaction is something a mirror can't deliver and is the reason why guinea pigs tend to lose interest pretty quickly.
Something that smells of the dead companion and that they can snuggle into is often more of a consolation for them during the first days. In the longer term, only another guinea pig can replace another one; no piggy is ever too old for that!
The oldest bereaved piggy I adopted was 7 years old and lived to celebrate her 9th birthday, simply because of the renewed zest for life from companionship. My current 8 year old lady loves to lie in the corner where her cage (in which she lives with a 3 year old 'husboar' and a half year old sow) meets two others; she's definitely a 'piggy soap' addict!