I've got 5 kids and we really stress communicating in our house. Yes you need to share but no you do not need to be the toy martyr.
this is what I do.
If a child- Joy has a toy and another one- Sam wants it- lets say they are trying to take it away- this is what I do (regardless of whose kid is which side of the toy war)
"Okay Sam and Joy calm down please and lets use our words"
"Sam, Joy was playing with that toy and it is not kind to snatch it away. I can tell you would really like to play with it.
"and Joy, I know you did not like it when Sam tried to take it but screaming at him is not okay"
"Joy please tell Sam "Sam please don't take my toy it hurts my heart...."
(JOY)-"Sam peas dona tay my toy it huwrts my har"
"And Sam you need to tell Joy you are sorry for snatching and you may ask her if you can have that toy when you are done- so Sam say "I'm sorry Joy for snatching. I really like that toy, may I have a turn when you are done with it?"
(SAM) "I sowwy Joy for satching. I wike dat toy- may I have a turwn when you are done peas?"
I added the kid talk assuming I am dealing with the 2-3 range. If on the two side I do most of the talking for them kwim?
Most often what happens is the child with the toy quickly becomes done and will give it to the other. On occassion I have set a timmer for 10 minutes and atthe sound of the ding they switch.
As far as handling the mama here is my two sence...
As a mom I appreciate support from other moms- we only see so many things so being in the mama game together is great. Sometimes though things may not seem roght simply from a lack of one person seeing the whole picture.
I would tell your friend when she comes over that you are really working with your little one on using his words and with socializing skills. So if she see you handle a sistuation a little differently that is why. She will ask for a for instance and you can say- well like with sharing this is what I want to do... and tell her something like what I typed above. I am sure she will be receptive. And use the same structure way of handling a sistuation like that should it occur.
OH and make sure you always verbally praise the kids
"great communication"
"I like the way you shared that toy when you were done with it Joy"
"Sam you are waiting so patiently- what a gentleman!"
hope that helps!