Not so much as [something] as [something else]

‘She sees him not so much as her uncle as her friend’ is a perfectly normal English sentence. So, too, is ‘He is not so much her friend as her uncle’. If you want to insert he is between as and her uncle, you can, but it's not necessary.

People often insert the word "but" or a similar word to give a better rhythm. For example:

She sees him not as her uncle, but as her friend.

The word "as" is being used in two ways here:

• the "not so much X as Y" construction
• in the phrases "as her uncle", "as her friend"

The question's example sentence is wrong because one "as" has swallowed another "as". Using brackets to indicate two parallel phrases, the grammatically correct form would be:

She sees him not so much [as her uncle] as [as her friend].

This is stylistically awkward because of the "as as". One way to remedy it is to take "as" out of the "not so much X as Y" construction, like this:

She sees him as not so much [her uncle] as [her friend].

I'm not saying that that construction's X and Y cannot be preposition-phrases. Rather, the awkwardness in this example comes from the prepositions being "as".