Calculating the concentration of product in a solution based on the peak area of HPLC Please help me resolving the following:

A sample solution, $\pu{0.24mg/mL}$ Compound A, is exposed to heat to form Compound B. From HPLC data report I have peak area for Compound A and Compound B.  The  STD  concentration is 0.3 Compound A. The concentration of Compound A in sample solution is $\pu{0.20mg/mL}$ Compound A calculated using STD Compound A information. Which is the correct way to calculate concentration of Compound B?

My advisor in the project I'm participating insist I must calculate the concentration of Compound B since is a degradation of Compound A  in term of the amount lost of Compound A is the amount gained by Compound B (she call it a indirect method for calculating the concentration). I'm at lost! Originally I performed the calculation using a STD Compound A at the LOQ level of the impurity:
$$\text{[Compound B]} = \frac{\text{Area Compound B} \times \text{[STD Compound A]}}{\text{Area STD Compound A}}$$
The new approach is calculate the [compound A] using a STD at the level of the sample and then indirectly calculate the [Compound B].
Your advisor is making a pretty dangerous assumption (although sometimes pharma people do it). The assumption is that the impurity is structurally similar to the main compound and its UV-Vis spectrum is almost similar i.e., their extinction coefficients are the same.
The idea in your formula is that the calibration curve for A will be same as B. Well that might be a first order approximation when the impurity is known to be structurally similar.
If B is a known compound, you should buy a separate standard, and make a calibration curve.