Occasionally pregnant mares will exhibit premature udder development and lactation. Several things may be occurring such as impending abortion, in utero death of one twin fetus, placental separation, or bacterial or fungal placentitis. Sometimes, a pregnant mare will, for unknown reasons, show mammary enlargement during mid to late gestation, which subsequently spontaneously regresses. A mare may not be pregnant and also have mammary development and milk secretion. This situation is referred to as "galactorrhea," the causes of which are not well defined. In humans, the condition is thought to be caused by elevated blood prolactin levels. Prolactin is one of the hormones responsible for milk production. To answer your first question as to whether she is pregnant, it is possible, but udder development can occur without pregnancy. Estrus should not affect milk production as the hormones involved in the estrous cycle of the mare are not the same as those involved in lactation. As regard to your second question, having a pregnant mare next to this mare should not affect her sympathetically nor have any other influence on this mare to cause her to have mammary development and milk secretion. My advice would be to have her checked for pregnancy. If she is not pregnant generally galactorrhea is not a problem for the mare and will resolve itself without any type of treatment.