If she's a multip with just a lip and at 0 I would get her sitting up if baby tolerates (ie doesn't get any worse) and try my damnedest to get that lip gone so she can push and have a baby. Inform your provider, especially if you are calling the FHR abnormal for recurrent complicated variables. The fastest way to resolve the problem is to deliver, right?
My comfort level and calling the doc depends on the depth and length more than anything. 90 Bpm for 30 seconds from a baseline of 120? Probably would just call and say "hey, just so you know, this is happening." 90 bpm for 50 seconds from 155? Then I would call and ask them to come lay eyes on it.
Also depending on the facility and provider, her MD could try to hold the lip back with a push and see if it stays gone. Most of the docs I've worked with would do that without hesitation for a multip and if the cervix is soft and stretchy it usually works. I've even done it at the docs request before but I would always wait for their guidance and discuss how to do it properly or you can make it swell and make things worse.
Does she have an epidural? Have you tried hands and knees, leaning forward etc? I love my upright positions, even with an epidural, and I find we tend to stay away from them for some reason. Everyone I work with now thinks their blocks will drop but I've rarely had that happen and it really helps to keep things going. Last night I had a primip with a couple of recurrent long 6-7 minute decels and they resolved in high fowlers so we kept her there. Couldn't turn the oxytocin back on but she was 8 cm when I left this morning!!!
But quite honestly I would just get rid of that lip and then have a baby