Probably very easily. If the suckler was awake, he or she could simply latch straight onto their partner’s waiting nipples, (assuming they were left exposed for the purpose). The nursing sensation would probably stir the woman awake, or she might just feel the accustomed pleasure in her sleep and barely stir at all, save for a contented moan and smile.
On the other hand, if the woman woke first, she would offer a breast to her partner’s mouth, gently pressing the nipple to their lips, which would stir them enough to latch automatically, even if still half asleep. I imagine this is how some couples practise night-time suckling while inducing, when the breasts must ideally be stimulated every 4 hours a day, including once during the night.