Sorry if this sounds like I'm just repeating myself but a violinist can play with just temperament in any key if he/she has sufficient skill. A guitar has to play with equal temperament all the time, however sweetened the tuning. The violinist has the potential ability to play in tune with the guitarist but this would require abandoning the true sweetness of just temperament.

Some guitarists draw out a chart of strings and frets and, using a good tuner, make a note of how in or out of tune the guitar is at each fret of each string in terms of plus or minus cents.

If you can be bothered to do this it might also be worth repeating the operation with the capo in different positions.

If you are into spreadsheets you can plot the blank chart in one and copy it into different pages for different capo positions.

If the results of this show that your guitar is reasonably in tune all over the neck then the problem is with the fiddle player. If the guitar is not in tune all over the neck you could show the results to a luthier or guitar tech and ask for advice.