Hello and welcome to HCM,
I dont see any reason to be afraid if you have only swapped around bass instruments.
Mono is an infectious disease and a person who has it carries the risk of transmitting the disease upto 18 months , highest chances are within 6 weeks of infection, which has surely passed with your friend.
The mode of transmission is saliva. And unless you have not come in contact with the saliva (kissing or a lot of sneezing
), there is nothing to worry about.
Hope I helped