Can someone still get pregnant after being diagnosed with endometriosis and one ovary?

Absolutely. One can conceive perfectly normally with only one ovary. While endometriosis can make it more difficult to conceive, it rarely absolutely prevents conception. If you lost one ovary because of enmdometiosis, be sure to have regular follow ups and ultrasous of the other ovary.
Yes. It is well known that endometriosis can cause infertility, however many women with endometriosis do conceive with no difficulty. Fertility can be improved with medical or surgical treatment of endometriosis, for those women with endometriosis who do not conceive. Having only one ovary may not have a big impact on fertility, as long as the fallopian tube and uterus are normal.