It's been regularly done for decades. Almost a century, in fact!

The Herzsprung-Russel diagram allows us to crossplot luminosity against colour temperature, which for main-sequence stars is a function of mass. (A statement which is well-proven by numerous measurements of the mass of binary systems).

Given this calibration, one can measure the mass of any star, solitary or not, providing it is on the main sequence and has been resolved as a discrete object whose spectrum (colour temperature) has been measured. This gives its mass, quite accurately.

Then, one compares apparent luminosity (light intensity), to absolute magnitude (light intensity at a standard distance) for that class of star, and finds its distance.

The only remarkable thing about this event is that the red dwarf is so distant that only Hubble could resolve it, whereas the more distant and vastly more luminous background star could be resolved (and thus weighed and measured) by a "normal" observatory.