I don't blame Sharon for being uncomfortable but she has to be fair. Jack has been great about Sharon's friendship with Brad. I believe Jack has grown to love Sharon and i KNOW Phyllis loves Nick, so I don't think she has anything to worry about.
Also, I get the feeling the frienship might be tested. When Phyllis finds out what Jack did to Gloria, she might see Jack very differently. Jack is her friend but Phyllis said Michael and Lauren are FAMILY!