camiAnana You should be able to achieve it by duplicating the right arm slot, placing it above the guitar in the draw order, and then editing the mesh attachment to make the mesh hull into the image of the arm so that you can hide the area you want below the shirt.
The actual operation will look like the video below:
The reason this trick works is that the duplicated mesh retains the same weight settings as the original. If you play an animation with the duplicated mesh, the vertices will be transformed in the same way as the original mesh, so there is no apparent misalignment and almost no discomfort should occur. However, it is not automatically linked to the original mesh, so if changes are made to the original mesh, you will need to do the same operation again.
I hope this will help you.