Would it be illegal ?
Unless there's a specific clause in the contract stating that you shall not disclose the details of your remuneration, there is no legal issue with saying "My salary is XXXX per annum", or "The standard starting salary is XXXX." This kind of clause can happen, but in my experience it's not a common one. If her contract has one of these clauses
Would it be taboo ?
This is both country and company dependant (please add a country tag).
In France, where I come from, you generally don't mention your income, or you stay really vague about it. However, a common indicator of revenue is the rent / location of your flat. In some other countries, there is no taboo about it.
In the same vein, I worked in market finance quite a bit, and I never heard a banker disclose his bonus. People were much more open to income disclosure when they worked in start-ups.
If the other person is a good friend of yours, he could do you the favour of checking internally the starting salary. Your current formulation is okay, but you are asking this person to challenge the offer his company has already issued, which might make him uncomfortable. You could simply say "Hey, I know someone who would be super interested in applying at your company, but he would like a rough estimate of the starting salary, would you mind inquiring ?" I would not mention that he the offer is already formulated.
Another option ?
This part might be slightly out-of-topic. You could use websites providing that kind of information. A number of them, such as Glassdoor, provide comparisons for similar positions in the company or in the field. If the company is large enough, you will probably have enough information to check whether his offer is actually low, or whether this particular firm doesn't pay well.