Use a clip-on mic attached to the bridge or a similar contact mic. Don't try and use anything acoustic for PA/FOH. SM57 is definitely not suitable for a 'cello under any circumstances.
There are quite a few piezo pickups available. Ideally you want something that is going to get excited only by the vibrations from the instrument and not from any ambient acoustic activity - this will help prevent any feedback. You want the greatest 'gain-before-feedback' ratio possible.
Typical mics that would work well with 'cello in a PA application would be: https://www.sweetwater.com/store/detail/VC1Pickup--kna-vc-1-portable-piezo-cello-pickup
Sounds to me like the 'cellist is bringing a lavalier mic and using some sort of foam insert to attach the mic to the 'cello. This is not a particularly good idea. as the mic will still be subject to external acoustic activity.
Being a lavalier, it will need both phantom power and a phantom power adapter for the lavalier. Most of the time, this will be provided by an XLR mic adapter that is specific for the lavalier, but you will need to ensure that +48v phantom is provided to the adapter.
From the Beyer specs...
"It delivers extremely natural, true sound reproduction without colouration
due to the proximity effect. The omni-directional polar pattern ensures even sound -
regardless of how the capsule is positioned.
For wired operation at a phantom powering input, the optional CV 18 is necessary."