So far I've really only mentioned the Westminster elements that might mean Ferrier may well struggle to effectively represent her constituents. But there are also issues in the constituency too.
In order to represent his or her constituents an MP must engage effectively with them, typically about 100,000 people. This is pretty well impossible on a direct one-to-one basis so MPs will use other routes. First an MP will use activists, which may also include other elected representatives - e.g. councillors, members of devolved assemblies etc as extra eyes and ears - knocking on doors, meeting people in a variety of situations. That knowledge will be fed back to the MP. When an MP is kicked out of her party they lose that base of activists unless they are more loyal to him or her than the party. In Ferrier's case, considering the misdemeanour she's been kicked out for I cannot image many activists being prepared to 'pound the streets' on her behalf, which would also involve having to step back from the SNP. Councillors, MSPs etc will not be allowed to formally support a non-SNP MP unless they also resign from the SNP, which I don't think any have. So her ability to engage by using extra 'eyes and ears' will be massively diminished.
Another common way in which MPs engage is through visits and meeting - opening a new school building, giving a short talk at the local business leaders forum, turning up at the local food bank etc etc. Under normal circumstances this is both good publicity (for both the MP and the organisations) plus gives a great opportunity for quiet conversations on matters of concern etc. I imagine those invites will have dried up - imagine the conversation amongst a school governing board:
'Who should we invite to open the new building?'
'Hmm - we could ask the MP'
'What, disgraced, convicted, double suspended Ferrier - not sure that's going to send a great message to the kids or parents'
'Oh yes, and we'd get terrible coverage in the local rag'
'And she's almost certainly not going to be our MP in a couple of months, so any messages we could get across will simply be lost'
'What's the name of our local MSP again?'
So no invite - and if she reaches out no doubt plenty of those requests will be quietly declined.
The lifeblood of engagement with constituents, involving activists and visits, meeting etc will be cut off for a disgraced, convicted, suspended MP.