Using the gerund to express causation [duplicate]

My case here is that I was writing something for school, and my teacher explained an error which I really can't see.

She referred to articles about "dangling participles", which was previously unknown to me. I find it hard to relate the examples I've found about it to my situation.

So this is a text I've written and the bold part is where the problem arises: (You could read only the bold part if you please)

Wandering, and lost in his thoughts, there is a deep despair occupying the mind of this man. He is barely aware of his surroundings, except for those few moments when the loud blissful voices break through the protective barriers, developed by his immense sadness and grave frustration, into his mind. ”How can they all possibly be so happy?” he thinks to himself. Staring blindly, with his head low, as he walks in a slow pace, whispering ”Mary, Mary, my Mary,” over and over again. Yesterday was her funeral. She had passed away from a severe type of cancer, at a relatively young age. It was his wife. They had been together since high school and were greatly attached to each other. Having spent nearly half his life by her side, we cannot but imagine his grief. Indeed, he could hardly picture a life without her. So, alone he roams in the midst of all the seemingly incessant cheering and noises, drowned by his sorrow.

Her comments on that part was:

we is slightly out of place here – see my comment below

These few sentences are about him and his emotions, so ‘we’ seems out of place. If the sentence starts: ‘Having spent nearly half his life by her side’, the reader expects it to continue: ‘he’. After all we have not spent half our lives by her side.

Is she right? If so, could anyone try to explain why?

The problem is that the modifier clause ("Having spent nearly half his life...") refers to one party (the man), while the subject of the sentence is a different party ("we"). To eliminate the dangling modifier, we could rewrite the sentence as:

We cannot but imagine the grief that he, having spent nearly half his life by her side, must have experienced.

...so that the modifier clause properly refers to the man rather than to us. But beware of revisions like:

Having spent nearly half his life by her side, his grief can be readily imagined.

...which commits the same sin: now his grief is the subject of the sentence, and his grief didn't spend half its life by her side, he did.

Having spent nearly half his life by her side, we cannot but imagine his grief.

The participle having does not have a noun nearby that it modifies. So there is no noun nearby that "has spent". In that case, this means the participle should modify the subject of the (main) clause. In your sentence, the subject is we. So then it would mean, "we have spent". But that is clearly not the intended meaning, because it is not we who have spent nearly half his life by her side.

So then what does the participle modify here? Nothing: it is dangling in the air.