how long should be waited between exit and re-entries on 10 years, B2 multiple entries visa?

[newbie to this]

I am new to this forum and hope you can help.
Thanks in advance .
My mother in law has an EU passport with a 10 years multiple entries B2 visa.
She is in the US now but her I-94 expires on June 10. Her outbond ticket is booked for the 11th. We just noticed that a couple of days ago.
we have 2 options... she is here to help with her daughter, my wife, who is a cancer patient and presently in the hospital. My Mother in law can go to Mexico or canada and come back riight away before the 10th so she remains legal and does not overstay her visa. If this is done she want to stay another 3 to six months to assit with my wifes care.
What is the best way to aproach this? we have only 10 days to get it resolved.
Please help...

 

[deepblue_NC]

I'm in a similar situation, my wife is also a cancer patient, and I'm trying to extend my mother-in-law's stay. I filed I-539 for that. I don't know the answer to your question but as far as I know visiting canada or mexico does not extend your I94. You may want to double check that before relying on that option. Good luck with everything.