Catch them at a neutral time, not right after a barking spell, and keep it friendly and specific. Something like, "Hey, I hope this isn't awkward, but I work from home and I'm hearing your dog bark a lot when you're out, usually late morning into the afternoon. I figured you might not know since you're not here, so I wanted to give you a heads up. Is there a way we could figure out a fix together? I'm happy to text you if I hear it ramp up." If you can, mention a couple exact days and times so it feels factual rather than a complaint, and keep the tone calm and collaborative.
Have a few practical ideas ready to show you want to help, not blame. If the dog is outside, suggest bringing them in during the day, closing blinds to reduce triggers, and leaving a fan, radio, or white noise on to mask hallway or street sounds. Enrichment helps a lot, so mention a stuffed frozen Kong, lick mat, or puzzle feeder right before they leave, and a midday dog walker or daycare a couple days a week if possible. If it started recently, suggest a vet or trainer check for separation anxiety and ask if they would be open to some gradual alone-time training. Offer to keep a short log for a week and share patterns, then agree on a timeline to try changes and check back, like "Let's see how it goes for two weeks and revisit."