To be clear: DO NOT use the standard Stable Diffusion 1.5 Inpainting model.. it's useless, trash, garbage, etc. The Stable Diffusion 2.x models are even worse, since they removed a lot of NSFW training data.
You need to select the URPM inpainting model (at the top left), as mentioned in the guide. You can also use "Realistic Vision 1.3 Inpainting" model available on Civitai.com, that one is also good for nude inpainting.
Additionally, if you don't give the app enough masking room, it won't work.. it can only 're-generate' the masked area.. so you need to mask a big area to let the model regenerate what is behind the mask, and let it work its magic. If you only mask a very small area, like only a 'tight' bikini, it won't work that great.
There were issues with this image (due to the lighting or something, not really sure) that caused a 'color distortion / difference' below the masked area.. it's still there, but a bit less since it masked more of the body.. i think the best way to solve this is to either mask her total body, or use another Sampling method. Below a full screenshot, including parameters (very basic) and the source & dest..
Result:
In most cases you will never replicate the full breast structure of the source image, due to the problems I listed above. So it won't be 100% perfect.