Context. A WIFE saying to her husband “you’ve let yourself go” can be just as dicey as a husband saying that to his wife.
An objective third party can say this stuff, though.
If you’re married and one problem is that your wife has let herself go — maybe this is a reason to pay for a counseling session. Then you have that buffer or intermediary.
Or, if the two of you have good open communications, you CAN tell her. Schedule a time where the two of you can discuss the marriage and let her know this is an issue. But be prepared that she may have her own issues with you, and working on these things is not a one-way street.
For example, if you’re unhappy that she’s let herself go but you’re coming home every day and parking yourself on the couch playing video games while she is doing everything — looking after the kids, cooking dinner, etc. — don’t be surprised if she tells you that she’ll need you to start doing some of that so she has time to hit the gym.
At any rate, I’m not into double standards. The needs of a man in the marriage are just as important as the needs of the woman. So having honest and open communication so both peoples’ needs are met — that’s really crucial.