How to inform adviser that morale in group is low?

You do it in exactly the same way as you raise most problems with most bosses:

You start by briefly celebrating something successful.

You go on to quickly point out one or more of their objectives that aren't being realised or won't be realised.

You concisely describe how the problem (in this case low morale) is causing that. You don't put blame on people. You just describe how the problem leads to the objectives not being met.

Then you succinctly give them some realistic, usable options to address the problem.

It will really help your case if you can demonstrate that you are aware of some of the pressures and conflicting demands on them.

And do have supporting evidence available to back up your statements, if required. Don't deploy it unless challenged - the goal is to keep things concise and focused.